1. Field of the Invention
The present invention relates to apparatuses and methods for controlling a backlight and liquid crystal displays, and more particularly, to a backlight control apparatus and method capable of preventing unevenness in luminance of a liquid crystal display when the display is viewed from an oblique angle, and the liquid crystal display.
2. Description of the Related Art
Liquid crystal displays (hereinafter, LCDs) each include a liquid crystal panel and a backlight arranged on the rear of the panel. The liquid crystal panel includes a color filter substrate having a pattern of red, green, and blue color filters, and a liquid crystal layer.
In each LCD, changing a voltage applied to the liquid crystal layer controls the orientation (twisted states) of liquid crystal molecules. White light, coming from the backlight and transmitting through the liquid crystal layer according to the controlled states of the molecules, passes through the red, green, and blue color filters to produce red, green, and blue light beams, so that an image is displayed.
In the following description, the above-described control of changing an applied voltage to control the twisted states of liquid crystal molecules and change transmittance will be termed “aperture ratio control”. In addition, the intensity of light which is emitted from a backlight, serving as a light source, and is incident on a liquid crystal layer will be called “backlight luminance”. Further, the intensity of light which emerges from the front surface of a liquid crystal panel and is perceived by a viewer visually recognizing a displayed image will be called “display luminance”.
In typical LCDs, while the whole of a screen of a liquid crystal panel is illuminated evenly by a backlight at a maximum backlight luminance, only the aperture ratio in each pixel of the liquid crystal panel is controlled to obtain the necessary display luminance in each pixel in the screen. For example, if the whole screen displays a dark image, the backlight emits light at the maximum backlight luminance. Disadvantageously, the power consumption is high and the contrast ratio is low.
To overcome the above-described disadvantages, for example, Japanese Unexamined Patent Application Publication Nos. 2004-212503 and 2004-246117 disclose methods of partitioning a screen into a plurality of segments and controlling the backlight luminance in each segment.
The above-described backlight control in each segment (hereinafter, referred to as “backlight partition control”) will now be described with reference to FIGS. 1 to 3.
FIG. 1 shows an original image P1 displayed on an LCD. The original image P1 includes an elliptical dark region R1 having the lowest display luminance in substantially the center of the image. The display luminance of the image P1 gradually increases with distance from the region R1 toward the periphery of the image P1. The rate of change in display luminance from the dark region R1 to the periphery in an upper portion of the image P1 in FIG. 1 is larger than that in a lower portion thereof.
FIG. 2 schematically shows the structure of a backlight.
Referring to FIG. 2, the backlight has a lighting area including segments arranged in six rows (extending in the horizontal direction)×four columns (extending in the vertical direction), i.e., 24 segments.
When the backlight emits light corresponding to the original image P1, the backlight reduces the backlight luminance (i.e., attenuates light or reduces the amount of light) in each of two hatched segments in accordance with the display luminance of the region R1 of the original image P1.
Consequently, a backlight luminance distribution shown in FIG. 3 is obtained on the basis of the original image P1 of FIG. 1. In this distribution, the display luminance is the lowest in a substantially central portion of the lighting area and gradually increases toward the periphery. As described above, partially reducing the amount of light emitted from the backlight can lower the power consumption, thus increasing the dynamic range of display luminance.
Since the number of segments in the lighting area is generally smaller than the number of pixels in the liquid crystal panel, the display luminance distribution of the original image P1 in FIG. 1 does not agree with the backlight luminance distribution in FIG. 3. There are many pixels having the difference between the backlight luminance and the display luminance. For instance, although pixels arranged on a line Q-Q′ of FIG. 3 have different backlight luminances, the corresponding pixels in the original image P1 have the same display luminance. In the backlight partition control, therefore, the aperture ratio in each pixel on the line Q-Q′ is set higher than that without the backlight partition control so that the amount of transmitting light is larger than that without the backlight partition control. In the following description, apparent display luminance obtained by aperture ratio control, i.e., changing the aperture ratio so as to compensate for controlled backlight luminance will be termed “corrected display luminance”.
FIG. 4 is a conceptual diagram showing the relationship between the backlight luminance and the corrected display luminance in the backlight partition control.
A backlight control unit for backlight partition control controls the aperture ratio in each pixel in a predetermined region so that the corrected display luminance distribution MCL is inverse to the backlight luminance distribution MBL in order to realize the same display luminance T0 in the predetermined region. In this instance, the level of corrected display luminance depending on how much the aperture ratio is changed is determined by the transmittance characteristic of liquid crystal shown in FIG. 5.
The transmittance characteristic of liquid crystal obtained when a screen of an LCD is viewed from the front is typically used as reference. The transmittance characteristic shown in FIG. 5 is also obtained when the screen is viewed from the front (hereinafter, also referred to as “when viewed from an angle of 0 degree”). This transmittance characteristic has been previously evaluated and determined.